It is well known to mix, at the transmitter of a microwave radio communications system using QAM (quadrature amplitude modulation), an IF (intermediate frequency) signal with an LO (local oscillator) signal to produce an RF (radio frequency) signal which contains sidebands above and below the LO frequency, referred to as the carrier frequency. Only a selected one of the sidebands is transmitted, the other sideband and the carrier frequency being suppressed. A double balanced image reject mixer is desirably used to facilitate this. Because the signal frequency is increased, this is referred to as an up-conversion.
With such systems increasingly using more QAM states and requiring increased dynamic range, there is an increasing need to improve signal-to-noise ratio. This leads to increased transmitter power levels. Consequently there is a need to improve the suppression of the suppressed sideband and carrier frequency components from the output of the image reject mixer, and hence from the signal to be amplified and transmitted. Because the carrier frequency is closer than the suppressed sideband to the transmitted, selected, sideband, and is at a relatively higher level due to the nature of the image reject mixer, the presence of carrier frequency components at the output of the frequency converter presents a particular problem.
This problem is exacerbated by the fact that, for linear operation of the image reject mixer, the IF signal must be supplied to the mixer at a much lower level than the LO signal. Consequently, carrier frequency components which appear at the output of the image reject mixer due to imperfect balance can have a magnitude which is comparable to that of the selected sideband RF signal.
In order to remove such undesired carrier frequency components, referred to below as carrier leak, before amplification of the RF signal in the transmitter's power amplifier, it is known to provide a filter between the output of the frequency converter and the input of the power amplifier. However, such a filter operates at microwave frequencies and must meet stringent requirements in order to pass the desired sideband while sufficiently suppressing the carrier leak, and accordingly is expensive.
An object of this invention, therefore, is to provide an improved frequency converter in which carrier leak is substantially reduced.